Battery-powered vehicles, e.g. utility-type or so-called "floor" vehicles such as forklift trucks and like industrial vehicles for shifting loads, removing stacked loads from a storing location, and for stacking loads, generally comprise an electric motor for driving the vehicle wheels and a suitable control system for increasing the voltage applied to this motor to increase the vehicle speed. The power source is a battery which may be charged during periods in which the vehicle is inactive.
It is known further to provide such vehicles with power-steering systems or power-assist steering arrangements to ease the steering operation and prevent tiring of the operator, especially since vehicles of this type must be maneuvered frequently in small spaces. Such power-assist steering systems can comprise a hydraulic arrangement which allows the wheels of the vehicle to be steered with or without assistance by a hydraulic medium.
In other words the device may comprise a hydraulic power arrangement having a pump driven by an electric motor which is connected across the battery and to a hydraulic medium. The pump feeds a hydraulic motor, e.g. a piston-and-cylinder arrangement, to displace the steering linkage in proportion to the rotation of a steering wheel or other control element so that all of the steering force need not be contributed by the operator.
Such a power-assist arrangement can be operated without the auxiliary force supplied by the hydraulic mechanism, either via a direct mechanical connection between the steering wheel and the steering linkage or by having the steering wheel operate upon a displacement-type pump which drives the hydraulic medium into a power cylinder connected to the steering linkage. In either case the motor-driven pump supplies the hydraulic medium under pressure to assist in the steering movement so that the force necessary to swing the linkage and the steerable wheels connected therewith need not be supplied exclusively by the operator.
The required steering force is, however, in many operating conditions relatively minimal. For example, when the vehicle is moving rapidly, little if any auxiliary force is necessary to assist in the steering movement which can be accomplished, even with large loads, easily by the operator. However, at relatively low vehicle speeds the steering action must overcome a frictional force and, especially with large loads, requires considerable force. To avoid tiring the operator, the power-assist device is especially useful in these operating states.
Thus, during part of the time at least in which the vehicle is used, it is not essential to have power-assist steering while at other times it is practically essential.
With battery-power vehicles an important economical consideration is to limit unnecessary drain upon the battery so that the current drain therefrom should be as little as possible. When, as has been the case customarily heretofore, the power-steering pump is continuously driven by its electric motor, there is an unnecessary drain during those periods in which the power-assist function is unnecessary.
To avoid this disadvantage it has been proposed to turn on the pump only in case of need, e.g. when the steering wheel is exposed, upon turning, to a steering force which exceeds a predetermined level (see German published application - Offenlegungsschrift No. 1 480 205).
The difficulty with this arrangement is that the threshold force on the steering wheel must be first reached before the pump is turned on and is capable of providing a power-assist function. For the driver or operator this is inconvenient since he first must apply steering forces up to the threshold level without assistance and must then respond in his steering movements to the substantially instantaneous commencement of the power-assist function during the steering operation.